1. Field of the Invention
The invention pertains to the art of material handling vehicles and particularly to logging vehicles of the type generally known as log skidders.
2. Description of the Prior Art
There are two general types of log skidders depending on the boom configuration and its function. The rotary boom skidder has a boom movable through an arc allowing the operator to reach to the right or left of the vehicle to pick up trees which are not in the direct path of the machine. These machines have considerable flexibility, but there is an inherent instability problem in trying to grasp trees which are not in the direct line of the machine.
Other skidders employ a rigid boom which does not rotate but may be pivoted in a vertical plane to position the grapple over the end of the log. Only the grapple rotates and the machine must be driven to the tree. There is not a stability problem because the axis of the tree is approximately in line with the natural stability axis of the machine when seized by the grapple.
Either type of machine will usually have a wench-drawn cable passing over a fairlead on the boom with chokers on the end of the cable for skidding logs which cannot be reached by the grapple.
With an articulated vehicle it is usually not too difficult to align the log with the axis of the skidder's rear section, but more often than not the operator must dismount the machine and rotate the grapple by hand so that it is properly aligned and can be lowered to grasp the log. On the average, it may take an operator thirty seconds to alight from the machine, walk to the rear, align the grapple, return to his seat and lower the boom in position for grappling the logs. Decreasing the grapple skidding time is obviously a desirable objective since it increases productivity of the logging operation.
In one case it has been proposed to mount the grapple on an hydraulic motor at the end of the boom which is controlled from the cab of the skidder to hydraulically swivel the grapple to lie transversely of the log. This arrangement however requires complicated valving and overload protective devices to insure against damaging the hydraulic motor and grapple when the log swings to the right or left of the line of travel since the operator cannot be responsible for watching the load while driving the skidder.